A Vote-Share Concentration-Based Axiomatic Characterisation of the Probability-Ratio Index of Electoral Competition
DOI:
https://doi.org/10.25071/1874-6322.40549Keywords:
Electoral competition, probability ratio index, concentration, consistency in aggregation, competitive indifferenceAbstract
We axiomatically characterise the probability-ratio index of electoral
competition, which is based on the vote shares of parties competing in an
election. It is the ratio between the probabilities that two voters drawn at
random with (without) replacement voted for different parties under actual
vote shares across the competing parties and under equal vote shares across
them. This vote-share concentration-based measure is characterised using
two simple axioms, consistency in aggregation and competitive indifference.
The former expresses the index as a weighted sum of competitiveness in
two-party elections. The latter is concerned with the redistribution of vote
shares across parties.
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